Antimony mercaptides have been proposed as stabilizers for vinyl halide resins to guard against degradation by heat during molding and working of the resins into useful articles. Prior art patents which disclose such antimony organic sulfur-containing compounds and their utility as stabilizers include U.S. Pat. Nos. 2,680,726; 2,684,956; 3,340,285; 3,399,220; 3,466,261 and 3,530,158. U.S. Pat. No. 3,887,508 is directed to improvements in the utilization of antimony organic sulfur-containing compounds in combination with metal carboxylates. As also described in U.S. Pat. No. 3,919,168, certain metal phosphates can be combined with antimony compounds to achieve long term heat stability improvements in vinyl halide resins. U.S. Pat. Nos. 4,029,618 and 4,158,640 further disclose synergistic heat stabilizer compositions of such antimony compounds as stabilizers and ortho-dihydric phenols. More recently in U.S. Pat. No. 4,440,674, improved antimony stabilizers were disclosed for the prevention of resin degradation by heat and shear.
As an overview to the patent technology in the area of antimony compositions in general, a number of patents have issued including: U.S. Pats. Nos. 2,306,731; 2,476,821; 2,556,420; 2,921,917; 2,934,548; 3,367,954; 3,432,534; 3,504,005; 4,010,104; 4,038,198; 4,115,352; 4,217,258; 4,221,687; 4,231,895; 4,256,618; 4,269,731; 4,279,806; 4,279,807; 4,287,118; 4,327,000; 4,336,168; 4,361,665; 4,367,304; 4,381,360; 4,391,757; 4,394,325; 4,396,552; 4,416,797 and 4,440,891. These patents demonstrate the field of antimony compounds or compositions in general and there is no intention by listing them in this group that they are all relevant to a person of ordinary skill in this art as background to this invention. Nevertheless, they are offered herewith in keeping with the duty of candor so that an independent determination may be made.
Notwithstanding the substantial amount of effort that has been directed to improvements in the art of antimony compositions especially suited for the heat stabilization of vinyl halide resins, a number of problems still remain. The problems that remain may be described as generally relating to the propensity of such antimcny compounds to somehow cause staining in a vinyl halide resin upon the application of heat. For instance, a compounded polyvinylchloride resin (PVC) containing an antimony stabilizer with the usual additives such as pigments, lubricants, and the like will stain during processing. Staining, as the term is understood in the trade, is the propensity of the white pigmented PVC formulation to turn slightly yellow or off-white. Presently, molded or extruded potable pipes that are made with such PVC formulations containing antimony stabilizers usually are slightly or moderately stained, depending upon processing conditions. The staining is believed to be attributable to a number of causes. For instance, antimony sulfides may be formed during the course of the processing and they may contribute to a yellow color in the compounded resin. Furthermore, it has been observed that antimony stabilizers produce staining in compounded PVC which has been pigmented with certain types of TiO.sub.2. Ultraviolet light is also known to stain molded PVC articles which have been made with antimony heat stabilizers. Another staining problem is believed attributable to a phenomenon known as "cross-staining" wherein an antimony compound will react with residual sulfides that may be present due to residual amounts of organo tin compounds left in the compounding equipment or molding dyes. The antimony sulfides thus produced by cross-staining present an unattractive staining or yellowing of the molded part. Thus, staining attributable to any one of a number of the above causes has made antimony stabilizer compositions less attractive at times than competitive materials in the marketplace such as tin heat stabilizers for PVC resins.